The oxidative capacity of airborne particulate matter has been correlated with the generation of oxidative stress both in-vitro and in-vivo. In recent years, epidemiological studies have associated damaged caused by cellular oxidative stress with several common diseases such as asthma, chronic obstructive pulmonary disease (COPD), Alzheimer's and other neurological diseases. However, these studies are based on daily exposure and averaged oxidative capacity of ambient particulate matter. Oxidative potential of the particles depends considerably on their chemical composition, more specifically on their redox active compounds, such as transition metals and quinones. It is well known that physical-chemical properties of ambient particles vary with emission sources and the extent of photochemical aging. Thus, we expect diurnal variations in the ability of particulate matter to generate reactive oxygen species and exert oxidative damage are expected. Currently, there are several chemical and in-vitro assays to determine the oxidative capacity of ambient particles. However, significant amounts of sample are needed to obtain a quantifiable response. Using the common collection devices, long sampling periods are needed, usually 24 to 48 hours. With these long collection periods, chemical properties of the particles may be altered and peak exposures hidden. Recent studies on the association between airborne particle exposures and adverse health effects identify short-term peaks in particulate matter exposures as important factors in health threat, especially in lung diseases. An epidemiological study of the effect of short-term exposure to peaks in particulate matter concentrations found that asthma symptoms were more highly associated with 1-h and 8-h maximum PM10 (particles with diameter <10m) exposures than with 24-h mean PM10 exposures. Based on these results, the development of instruments capable of measuring exposure to peak concentrations of health stressors is of vital importance. In this project we propose a new approach for an on-line monitor of the oxidative capacity of aerosols (o- MOCA). The main objective is to develop a field-deployable system that allows in-situ, time-resolved assessment of the capacity of airborne particles to generate ROS. Our approach capitalizes on our firm's new particle growth technology that enables direct particle deposition into liquids, obtaining concentrated suspensions ready for chemical and in-vitro assays. The aerosol collector uses the water condensational growth technology which allows the collection of particles as small as 10 nm into concentrated water suspensions with efficiencies >90%. The oxidative potential of the collected particles will be measured using the chemical assays commonly known as the DTT assay (dithiothreitol assay). The o-MOCA approach has several advantages over the existing laboratory and on-line systems: i) it can efficiently collect PM2.5 directly into a small volume of water, increasing the particle concentration and thus reducing the time needed for collection; ii) with direct collection it avoids the most common artifacts associated with other particle collectio systems; iii) it allows for time-resolved collection and in-field direct analysis, allowing for a mre satisfactory daily characterization of the PM oxidative capacity. The ability to characterize the oxidative potential of aerosols in a time-resolved manner will provide more accurate results when assessing possible adverse outcomes related to oxidative stress responses resulting from PM exposure. Our goal will be achieved by completing the following specific aims: i) design, construction and off-line optimization of the chemical assay module; ii) interface the chemical module with the liquid collector; iii) conduct laboratory controlled studies to evaluate whether ou approach allows for near-real time measurements of the oxidative capacity of ambient PM.